Measurement of network characteristics can be performed using methods which include active probing of the network, i.e. injecting dedicated probe packets for the sole purpose of the measurement method. One example is disclosed in the U.S. Pat. No. 6,868,094 wherein an IP performance monitoring method is shown. In the US patent a timing probe data packet containing a send time stamp is sent over the network from a sender to a receiver. A receive time stamp is written into the probe packet at the receiver. The probe packet is echoed by the receiver and the probe packets sender performs an analysis based upon the send stamp and receive stamp. Another example is the BART method for available bandwidth estimation, developed at Ericsson AB. Aspects of BART has been published at several conferences such as:                [1] S. Ekelin and M. Nilsson, “Continuous monitoring of available bandwidth over a network path”, 2nd Swedish National Computer Networking Workshop, Karlstad, Sweden, Nov. 23-24, 2004.        [2] S. Ekelin, M. Nilsson, E. Hartikainen, A. Johnsson, J.-E. Mångs, B. Melander and M. Björkman, “Real-time measurement of end-to-end available bandwidth using Kalman filtering,” in Proc. 10th IEEE/IFIP Network Operations and Management Symposium, 2006.        [3] E. Hartikainen and S. Ekelin, “Tuning the Temporal Characteristics of a Kalman-Filter Method for End-to-End Bandwidth Estimation,” in Proc. 4th IEEE/IFIP Workshop on End-to-End Monitoring Techniques and Services, 2006.        [4] E. Hartikainen and S. Ekelin, “Enhanced Network-State Estimation using Change Detection,” in Proc. 1st IEEE LCN Workshop on Network Measurements, 2006.        
In an active probing method, each probing packet receives a time stamp as it is sent from the sending node. When reaching the receiving node it is time-stamped again, and the time stamps are fed into an algorithm in order to calculate an estimate of the characteristics metrics. When estimating available bandwidth, also the sizes of the probe packets are needed by the algorithm.
The probing packets are normally sent in pairs or in packet trains. The number of packets in a train may vary according the probing method. However, a train must consist of at least two packets. BART is essentially a packet-pair method, and in the cases where probe packet trains of length N>2 are used, the train can essentially be seen as a sequence of N−1 probe packet pairs, where all the interior packets double as being the first in one pair and the second in another.
The time interval between sent probe packets is selected by the probing algorithm in order to achieve a specific probing load on the network path. This time interval is related to the probe packet size; a smaller time interval between packets with a given packet size will give a higher probing load.
One reason for using pairs (or trains) as opposed to sending isolated probe packets is that the need for synchronization between sender and receiver is eliminated.
In the basic version of BART, no feedback from receiver to sender is assumed. The algorithm can be considered to be running at the receiver, and all input needed must then be accessible at the receiver.
Probing can be used in different ways. One possibility is probing end-to-end, i.e. from one sending host to one receiving host. Another scenario is probing edge-to-edge, i.e. from one aggregating node to another aggregating node.
One problem is that active probing increases the traffic load on the network, since probe packets normally don't carry any payload. If the use of probing-based measurement methods starts to grow, this could cause significant volumes of extra network traffic.